1. Field of the Invention
The present invention relates to orthogonal frequency division multiplexing (OFDM) transmission and reception devices and methods thereof. More particularly, the present invention relates to OFDM transmission and reception devices which transmit and receive OFDM symbols having a variable data transmission rate, and to methods thereof.
2. Description of the Related Art
With the development of electronic and communication technologies, digital technologies have been introduced into the field of broadcasting systems, and diverse standards for digital broadcasting have been published. Specific examples of such standards are the U.S.-oriented Advanced Television Systems Committee Vestigial Sideband (ATSC VSB) standard and the European-oriented Digital Video Broadcasting-Terrestrial (DVB-T) standard. These two standards differ from each other in many ways, such as the manner of audio compression and the channel bands employed. In particular, ATSC VSB standards employ single carrier schemes, but DVB-T standards employ multiple carrier schemes.
The multiple carrier schemes employed in DVB-T standards is an orthogonal frequency division multiplexing (OFDM) scheme. OFDM schemes are utilized as standards for IEEE 802.11a, ETSI BRAN'S HIPERLAN 2, European digital audio broadcasting (DAB) and digital TV DVB-T. A conventional single carrier transmission scheme in which information is carried by a single carrier causes interference between symbols to increase, so distortion also increases. Accordingly, an equalizer of a receiver must be complicated. In order to solve these problems of the conventional single carrier transmission scheme, OFDM schemes have been introduced.
OFDM schemes enable data to be transmitted using multi-carriers. Such OFDM schemes are able to convert data symbols input in series into parallel data symbols, to modulate each of the parallel symbols into a plurality of tone signals which are orthogonal to each other, and to transmit the modulated signals.
OFDM schemes have been widely applied to digital transmission technologies, such as Digital Audio Broadcasting (DAB), digital television, Wireless Local Area Network (WLAN) or Wireless Asynchronous Transfer Mode (WATM). In particular, OFDM schemes maintain orthogonality between tone signals, unlike conventional multi-carrier schemes, so it is possible to obtain optimum transmission efficiency during high speed data transmission. Additionally, almost the entire available frequency band can be utilized and multi-path fading can be reduced.
In a DVB-T system, pilot tones are inserted into OFDM symbols, and the OFDM symbols into which the pilot tones are inserted are transmitted to a reception device, so that the reception device may perform channel equalization and estimation. The number of pilot tones to be used may be determined according to the type of standard. For example, 142 pilot tones may be used in a 2K Fast Fourier Transform (FFT) transmission mode, and 568 pilot tones may be used in an 8K FFT transmission mode. The capacity of pilot tones corresponds to approximately 8.3% of the total data transmission capacity.
The reception device checks the pilot tones inserted into the OFDM symbols, to analyze the noise of channels, so as to perform channel equalization. The pilot tones are inserted into the OFDM symbols according to regular pilot insertion patterns, which are shared between the reception device and transmission device.
A conventional DVB-T system has the advantage that it is able to receive signals while moving, because channel estimation information is rapidly updated by frequent insertion of pilot tones in a time shaft. However, excess pilot insertion causes a reduction in the capacity of data to be actually transmitted.
If a reception device is fixed in place so that it hardly moves, there are not many changes in the time characteristics of channels formed between the reception device and transmission device.
However, even in this situation, pilot tones are inserted into OFDM symbols in the conventional DVB-T system without changing the pilot insertion pattern, so the data transmission rate is reduced unnecessarily.